In the context of the present patent application, a “thin film” will be a film exhibiting a thickness of less than 5 μm.
A photovoltaic module comprises several solar cells placed in series. This is because the electric voltage generated at the terminals of just one solar cell, which is less than 1 volt, is generally too low for many devices. It is therefore necessary to place a large number of cells in series. Thus, the voltage delivered by a photovoltaic module is of the order of 100 volts, for approximately 100 cells connected in series.
For thin film photovoltaic modules, this placing in series can be obtained by etching and deposition stages carried out on one and the same substrate. A monolithic interconnection is thus produced. This exhibits a considerable advantage with respect to the conventional technology of bulk crystalline silicon. This is because the preparation of modules made of crystalline silicon requires profound and laborious wire connection and welding operations. All these operations are rendered unnecessary with the thin film technology.
The process for the monolithic interconnection of the thin film solar cells requires three etching stages, conventionally named P1, P2 and P3.
The first stage (P1) ensures electrical isolation of two adjacent cells at the rear-face electrode of the solar cells.
The second stage (P2) makes it possible to connect the front-face electrode of a given cell to the rear-face electrode of the adjacent cell.
The third stage (P3) consists in electrically isolating two adjacent cells at the front-face electrode.
Various techniques are employed to carry out this monolithic interconnection process.
The most conventional techniques are mechanical etching or laser ablation.
Reference may thus be made to the document U.S. Pat. No. 4,502,225, which describes a device comprising an etching tip intended for semiconducting devices.
The use of laser in thin film solar cells is described in particular in the papers “Selective ablation of thin films with short and ultrashort laser pulses”, Hermann et al., Appl. Surf. Sci., 252 (2006), 4814, or also “Laser applications in thin-film photovoltaics”, Bartolme et al., Appl Phys B, 100 (2010), 427-436.
These etching techniques exhibit the advantage of being able to be employed for a great variety of materials deposited as thin films, such as, for example, CdTe, a-Si, CZTS (of general formula Cu2ZnSn(S, Se)4) or CIGS (of general formula Cu(In, Ga) (S, Se)2).
However, these etching techniques each exhibit disadvantages.
Thus, mechanical etching results in damage to the materials as a result of the presence of mechanical stresses on the films, in the formation of debris on the surface of the films close to the etching line, which can result in short-circuiting problems, and in the wear of the etching tips. Furthermore, generally, the quality of the mechanical etching is very sensitive to many parameters, such as the morphology or the properties of the thin films, and also to the operating parameters of the etching tips.
Furthermore, laser ablation is not simple to carry out. This is because it would be found that the material withdrawn may melt and partially fill in again the groove produced by the laser ablation. Thus, this technique does not make it possible to obtain a clean surface necessary to produce an electrical contact of good quality.
Use may also be made of chemical etching methods. However, these methods are more complicated and more expensive to carry out than the conventional mechanical etching or laser ablation methods.
In order to put the invention better in context, a conventional monolithic interconnection process for a thin film photovoltaic module will now be described with reference to FIGS. 1a to 1f. All these figures are cross sectional views and represent different stages of the implementation of this process.
FIG. 1a represents a flexible or rigid substrate 1 which can be made of various materials, in particular of glass, or also of plastic or of metal (for example steel, aluminum or titanium).
Generally, this substrate is made of soda/lime glass, the thickness of which is a few millimeters and typically between 1 and 3 mm.
A molybdenum film 11, the thickness of which is generally between 100 nm and 2 μm and preferably of the order of 1 μm, is deposited on this substrate 1.
This molybdenum film will be used to form the rear-face electrode of the various cells forming the photovoltaic module.
FIG. 1a shows that an etching stage is carried out after the deposition of the Mo film. As indicated above, this etching is generally carried out either mechanically or by laser ablation. It results in the formation of a groove 110 devoid of molybdenum.
This groove 110 makes it possible to define the rear-face electrodes 11a and 11b of the adjacent cells 2 and 3 illustrated in FIG. 1f. 
This etching stage corresponds to the stage P1 mentioned above.
The width of the groove 110 is generally between 10 μm and 100 μm and it is preferably of the order of 50 μm.
FIG. 1b illustrates another stage of the process in which a photovoltaic film and, by way of example, a crystalline CIGS film is produced. This film has a light-absorbing role.
This stage consists first of all in introducing, onto the rear-face electrode 11, Cu, In and Ga metal precursors and elements of Se and/or S type, used for the growth of the CIGS film, a p-type semiconducting material.
Numerous deposition processes suitable for thin films can be used.
They can be vacuum processes, such as evaporation or cathode sputtering, or processes carried out at atmospheric pressure, such as electrodeposition, screen printing, doctor blading, inkjet printing or slit coating.
Thus, Cu, In and Ga precursors can be deposited by cathode sputtering. An Se and/or S film can subsequently be deposited on the stack obtained by a vacuum method or a method carried out at atmospheric pressure.
Generally, a bulk contribution of S or Se is always necessary. The chalcogen S or Se can be introduced in the elemental gas form, in the gas form (H2S or H2Se) or in the form of a film of evaporated S or Se deposited on the surface of the film of metal precursors.
It should be noted that the gases H2S and H2Se are highly toxic, which greatly complicates the use of these gases on the industrial scale.
The thickness of this film of metal precursors is generally between 300 nm and 1 μm.
The conversion of the constituents into a film 12 of crystalline CIGS is carried out by a high-temperature annealing, denoted selenization/sulfurization annealing, using a temperature rise gradient of between 1° C./s and 10° C./s.
Reference may in particular be made to the document U.S. Pat. No. 5,578,503, which describes a process for producing a semiconductor of the CuXY2 type where X is In and/or Ga and Y is Se or S.
The temperature is generally between 400 and 600° C.
The film of constituents can be covered with a cap, preferably made of graphite. This cap makes it possible to ensure a greater Se and/or S partial pressure during the annealing, which results in the diffusion of Se and/or S into the metal precursors being increased.
FIG. 1c shows another stage of the implementation of the process, in which a film 13 of semiconductor of n type is deposited on the CIGS film, in order to form the pn junction.
This film can be deposited by a chemical bath, by cathode sputtering or also by evaporation.
It can, for example, be composed of CdS and be deposited by a chemical bath, the film 13 exhibiting a thickness of a few tens of nm.
Other materials can be used, such as ZnS or ZnOS, for a thickness, for example, of between 5 nm and 30 nm.
FIG. 1c also illustrates another stage of the process which is optional. This stage consists in depositing a film 14 of intrinsic ZnO, the role of which will be explained later.
This film 14 is highly transparent in the solar spectrum and highly resistive. It is generally deposited by cathode sputtering and exhibits a thickness of a few tens of nm.
It may be noted that the film 13 prevents reactions between the ZnO and the CIGS and thus protects the film 12 during the deposition of the film 14.
FIG. 1d illustrates a stage of implementation of the process in which another etching is carried out, either mechanically or by laser ablation.
This etching, corresponding to abovementioned stage P2, consists in removing all the films deposited beforehand on the molybdenum film 11. This etching thus makes it possible to produce an opening referenced 111 in FIG. 1d. It will make it possible to produce a portion of the electrical interconnection between two adjacent cells.
The width of the opening 111 is generally between 50 μm and 150 μm and it is preferably equal to approximately 100 μm.
Furthermore, the distance between the openings 110 and 111 is generally between 50 μm and 150 μm and it is preferably equal to approximately 100 μm.
FIG. 1e illustrates yet another stage of implementation of the process, in which a film of a conducting transparent oxide 15 is deposited.
This film can be deposited by cathode sputtering and can exhibit a thickness of a few tens of nm.
It can in particular be Al-doped ZnO, exhibiting a thickness of approximately 500 nm.
This Al-doped ZnO film will be used to form a conducting transparent electrode referenced 15a for the front-face electrode of the cell 2 and 15b for the front-face electrode of the cell 3 (see FIG. 1f).
It is generally accepted that the film 13 of semiconductor of n type may exhibit discontinuities. The ZnO film 14 then has the role of ensuring electrical isolation between the conducting film 15 and the CIGS film 12.
Other materials, such as tin-doped indium oxide (ITO), silver nanowires or carbon nanotubes, might also be employed to produce this conducting transparent electrode. Likewise, other deposition techniques might also be used.
It is understood that the distance between the openings 110 and 111 has to be sufficiently high to prevent an excessively high interconnection resistance between the front-face electrode 15a of the cell 2 and the rear-face electrode 11b of the cell 3.
FIG. 1f illustrates a final stage of the process, in which another etching is carried out in the stack of films in order to definitively isolate the cell 2 from the cell 3.
This etching stage corresponds to the abovementioned stage P3. It can be carried out mechanically or by laser ablation and consists in removing all the films deposited on the rear-face electrode 11b. 
The opening 112 obtained makes it possible to electrically isolate the two cells 2 and 3 at their front-face electrodes 15a and 15b. 
The opening 112 more generally exhibits a width of between 10 μm and 200 μm and it is preferably of the order of 100 μm.
FIG. 1f also illustrates the pathway of the charges between the two adjacent cells 2 and 3.
Thus, the front-face electrode 15a of the first cell 2 makes it possible to collect, on the front face, the electric charges generated in this cell 2 and to convey them to the rear-face electrode 11b of the adjacent cell 3.
Due to the disadvantages exhibited by the conventional etching techniques, solutions have been provided in the state of the art.
They concern the stage P2 and they have the object of locally increasing the conductivity of the CIGS material in order to bring about the conduction of the charges from the front-face electrode of a given cell to the rear-face electrode of the adjacent cell.
It can be a laser treatment which makes it possible to locally bestow the metallic behavior on the CIGS. Reference may in particular be made to the paper by Westin et al., “Laser patterning of P2 interconnect via thin-film CIGS PV modules”, Solar Energy Materials and Solar Cells, 92 (2008), 1230.
Thus, the laser treatment makes it possible to create a region of greater conductivity than the remainder of the CIGS film which replaces the direct contact between the Al-doped ZnO film and the rear-face molybdenum electrode.
However, the lasers which can render the CIGS conducting are different from those used to etch it, during the P3 stage. Reference may in particular be made to the paper “Application of a Pulse Programmable Fiber Laser to a Broad Range of Micro-Processing Applications”, Rekow et al., NRC Publications Archives. Thus, two types of laser are then necessary, which considerably increases the production costs.
Metal precursors deposited locally on the rear-face electrode can also play this role by diffusing into the CIGS film during the growth of the latter at high temperature. Reference may in particular be made, in this regard, to the document US-2010/0000589.
However, this technique also exhibits disadvantages. This is because it requires first of all the addition of a supplementary stage of deposition of strips of doping elements, in comparison with the conventional monolithic interconnection processes. This renders the process more complex and increases the production cost.
Furthermore, the diffusion of the doping elements within the CIGS results locally in the formation of a modified semiconductor which exhibits a greater conductivity than the CIGS but a lower conductivity than that of a metal. Thus, the electrical connection between the front-face electrode of one cell and the rear-face electrode of the adjacent cell will be made through a modified semiconductor, the resistance of which is greater than that of a metal. The electrical connection will thus be of poorer quality than that which can be obtained by a conventional interconnection process since, in this case, the electrical connection is made through a conducting and transparent electrode in direct contact with the metal molybdenum electrode.